This invention primarily relates to the field of fixed-ratio DC-AC-DC converters, or DC transformers. It also relates to the field of AC-AC converters.
One of the main objectives of DC-DC converter design is to reduce size and weight without adversely affecting other constraints. Compact designs require small parts, which implies low energy storage.
Simple unregulated square-wave DC transformers require minimal energy storage in their magnetic and capactive components, but they have two main disadvantages. Substantial electromagnetic interference can be created by the steep slope of the voltage waveforms and by spikes on the input current caused by overlap in the conduction of the switching devices. Normal variances between individual switches and rectifiers produce dissymmetry in the transformer voltages and currents, and can cause the transformer core to saturate.
All variable-ratio switching converters must store considerable energy in their magnetic and capacitive components. Integrated magnetics and coupled-inductor converters, as exemplified by the teachings of U.S. Pat. No. 4,274,133 to Cuk et al., are among the most compact variable-ratio designs. Even though these topologies save space by placing two or more inductors on a common magnetic core, fluxes produced by the input and output windings add, thus increasing the energy storage.
Resonant techniques have also been used in the prior art to reduce component size, but the amount of energy storage required to maintain oscillations is substantial. Conventional resonant circuits store energy during some parts of the cycle and release energy during other parts. Enough energy must be stored to maintain oscillation for the greatest anticipated load. In the case of a parallel-resonant tank, this means that the current which circulates between the resonant elements is typically greater than the current which flows through the load.
U.S. Pat. No. 4,630,005 to Clegg et al. describes a current-fed, parallel-resonant inverter circuit in which, as a matter of economy, a small inductor used to provide a constant base current is placed on a common magnetic core with the main input inductor. The constant base current is actually achieved by developing a constant voltage across a resistor.
In accordance with the present invention, the input and output inductors of a current-fed, parallel-resonant DC-DC converter are placed on a common magnetic core. This arrangement produces two advantageous and unexpected results. First, when the output current is passed through a separate winding on the input inductor, it essentially cancels the DC flux produced by the input current. This greatly reduces the need for energy storage in this component, thereby reducing its required size.
When used in the topologies of the present invention, the two coupled inductors form a structure which is hereinafter referred to as a complement transformer. Complement transformers, like ordinary transformers, operate with AC voltages, and need to store little energy. Unlike ordinary transformers, complement transformers operate with DC currents instead of AC currents, and are intended for ripple cancellation instead of being used for transferring power.
The main transformer in current-fed, parallel-resonant circuits is connected in parallel with one or more capacitances to form a tank circuit. The second beneficial result of placing the input and output inductors on a common core is that the symmetry of the resulting DC-DC converter produces a unique condition in which the tank can freely oscillate with a small resonant current, independent of whatever load currents are applied. The magnetizing inductance of the main transformer can therefore be made relatively large, and the parallel capacitance can be made relatively small, both resulting in minimal energy storage. This condition is hereinafter referred as complementary resonance.
An analysis of complementary resonance led to the discovery of a more general condition that is hereinafter referred to as complementary conversion. In complementary conversion topologies, the input current flows through a series connection of windings of the complement and main transformers, and the output current also flows through a similar set of series-connected windings. These series connections allow an AC voltage waveform of any desired shape to be impressed upon the main transformer without dissipating a significant amount of power. This property allows the tank to oscillate freely in complementary-resonant converters.
The present invention comprises several alternative, but related, complementary converter topologies in which the low ripple characteristic of integrated-magnetics converters and the efficient switching characteristics of zero-voltage-switching resonant converters are achieved, while requiring only the modest energy storage of square-wave converters.